I just bought a used D700 and I don't think I'm crazy (my wife thinks I'm insane).

It's a very viable camera. If you don't need video and you aren't printing out huge movie posters, the camera will serve you well for years to come. I had a D300 so it's essentially the same mega-pixels and I was VERY happy with the D300. I just needed a larger viewfinder for my MF lenses and wanted to use the FX lens range more effectively.

The D700 is one of the best photo cameras out there in terms of build and interface. I don't think the D600 will be in the same class as the D700...it may have more bells and whistles and be newer but it won't be the beast that the D700 is.